1. Field of the Invention
The present invention relates to a facsimile apparatus, printer or similar image forming apparatus and more particularly to an image forming apparatus capable of controlling the conveyance of a paper sheet or similar recording medium.
2. Description of the Background Art
It is a common practice with an image forming apparatus to form a toner image on a photoconductive drum or similar image carrier and then electrostatically transfer the toner image to a paper sheet, OHP (OverHead Projector) film or similar recording medium (sheet hereinafter). The toner image transferred to a sheet is simply retained on the sheet by an electrostatic force and must therefore be semipermanently fixed thereon by heat and/or pressure.
Among various fixing systems known in the art, a fixing system of the type using a heat roller is predominant over the others because it has high thermal efficiency and safe. More specifically, in this type of system, two rollers, one of which is implemented as a heat roller, are pressed against each other and convey a sheet carrying a toner image thereon via a nip between the rollers. As a result, the toner image is fixed on the sheet by heat and pressure.
The toner image may be a monochromatic image or a color image formed by toners of different colors superposed on each other. A color image generally refers to a two-color or a three-color image. A fixing condition, i.e., the amount of heat necessary for fixation differs from a monochromatic image to such a color image. A full-color image, for example, sometimes needs a great amount of heat because a glass transition point matching with required gloss and coloring property must be established. In addition, a great amount of head is sometimes required in relation to the material and thickness of the sheet as well. In light of this, when the amount of heat of a fixing device and the increase of the same are fixed, it has been customary to vary sheet conveying speed for thereby increasing the duration of sheet heating.
To increase the amount of heat to act on a sheet carrying a full-color image thereon, the conveying speed of an image transferring device and that of a fixing device may be varied after the transfer of the image to the sheet, as taught in, e.g., Japanese Patent Laid-Open Publication Nos. 5-265299, 5-100515, 6-348146 and 8-22203. In accordance with these documents, the conveying speeds of image transferring device and fixing device are switched in matching relation to the number of toners and material, e.g., an OHP film poor in coloring property or wax-containing toner poor in gloss, so that a necessary amount of heat can be implemented. This scheme, however, has the following problems left unsolved.
A tandem, color image forming apparatus, for example, includes a plurality of photoconductive drums and a plurality of developing devices adjoining the drums. While these members each are driven by a respective motor, such motors are not controlled independently of each other, but are controlled in accordance with the overall image forming sequence. Consequently, when the speeds of the image transferring device and fixing device are varied, they influence the other drive sources. It is therefore necessary to use special arrangements for coping with a plurality of motor outputs, as distinguished from a single standard speed. Particularly, when a recording medium of the kind needing a great amount of heat for fixation is used, it must be conveyed at low speed. This cannot be done without resorting to a motor capable of outputting high torque inversely proportional to the low speed, as will be apparent from a relation between the rotation speed and the torque. Such an arrangement therefore increases the cost of the apparatus. Moreover, despite that only the image transferring device and fixing device are the subject of speed control, even a sheet feeding device must be variable in speed. However, the speed of the sheet feeding device cannot be varied until the pay-out of a sheet ends, resulting in a time loss.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-140845 and 2001-354337 as well as in Japanese Patent No. 2,915,510.
It is an object of the present invention to provide an image forming apparatus capable of rapidly varying the conveying speed of a recording medium without increasing cost or space.
An image forming apparatus of the present invention includes an intermediate image transfer belt to which a toner image is to be transferred from an image carrier, an image transferring device for transferring the toner image from the intermediate image transfer belt to a sheet, and a fixing device for fixing the toner image on the sheet. When the length of the sheet in the direction of conveyance is smaller than a distance between the image transferring device and the fixing device, a matching circuit varies only the speeds of the image transferring device and fixing device, but does not vary the speed of a device that executes a step preceding the image transfer step. The matching device then sets an interval between consecutive sheets matching with the varied speeds of the image transferring device and fixing device, thereby matching opposite sides with respect to the image transfer step to each other as to the number of sheets to be conveyed.